Optical direct amplifier device

ABSTRACT

An optical direct amplifier device can detect failure of an excitation light source without using an electric circuit even when a monitoring system of the optical direct amplifier device based on a line turning back is employed. The optical direct amplifier device includes first and second optical amplifiers respectively amplifying signal lights of first and second transmission path. The optical direct amplifier device has a monitoring circuit branching only monitoring light wavelength of different from a wavelength of the signal lights and extracting means for extracting a part of an excitation light incided to the optical amplifiers.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to an optical directamplifier device. More particularly, the invention relates to amonitoring circuit of an optical direct amplifier device.

[0003] 2. Description of the Related Art

[0004] In the conventional optical direct amplifier device, a rear-earthadded fiber 1 a, a WDM (Wavelength Division Multiplexing) coupler 3 a,an optical isolator 4 a, an optical wavelength multiplexer/demultiplexer5 a are connected in sequential order from an uplink signal input end toreach an uplink signal output end. Similarly, a rear-earth added fiber 1b, a WDM coupler 3 b, an optical isolator 4 b, and an optical wavelengthmultiplexer/demultiplexer 5 b are connected in sequential order from adownlink signal input end to reach a downlink output end.

[0005] Laser diodes (LD) 6 a and 6 b are connected to a control circuit8 so that outputs thereof are controlled by the control circuit 8. Thelaser diodes 6 a and 6 b are connected to two input ends of the opticalwavelength multiplexer/demultiplexer 7. Two output ends of the opticalwavelength multiplexer/demultiplexer 7 are connected to the other inputends other than input/output paths of the WDN couplers 3 a and 3 b,respectively.

[0006] On the other hand, two branched ports of the optical wavelengthmultiplexer/demultiplexer 5 a are connected to respective opticalwavelength multiplexers/demultiplexers 10 a and 10 b. Also, two branchedports of the optical wavelength multiplexer/demultiplexer 5 b areconnected to respective optical wavelength multiplexers/demultiplexers10 a and 10 b. And, branched ports of optical wavelengthmultiplexers/demultiplexers 10 a and 10 b are connected with each other.In an up-link circuit and a down-link circuit of the foregoing opticaldirect amplifier, connecting portions of the optical wavelengthmultiplexers/demultiplexers 5 a and 5 b located downstream side of theoptical isolators 4 a and 4 b form the monitoring circuit.

[0007] Monitoring of the optical direct amplifier device in an opticaldirect amplifier system mounted the monitoring circuit employs a methodto transmit a monitoring signal from a terminal station for turning backa part of an optical signal within the optical direct amplifier deviceto an opposite side line (e.g. from the up-link line to the down-linkline) and to receive the monitoring signal turned back and returned bythe terminal station. In general, it has been employed a method fordetecting the optical direct amplifier device in abnormality of opticaloutput based on lowering magnitude of the monitoring signal output ofthe optical direct amplifier device in a monitoring signal wavelength.

[0008] However, in the monitoring circuit of the conventional opticaldirect amplifier device, when one of the laser diodes 6 a and 6 b inFIG. 5 is turned OFF, lowering magnitude of the monitoring signal outputof the optical direct amplifier device can be small to cause difficultyin detecting abnormality of optical output in the optical directamplifier device in certain characteristics of the rare earth addedfibers 1 a and 1 b or certain monitoring signal wavelength.

SUMMARY OF THE INVENTION

[0009] The present invention has been worked out in view of the problemset forth above. Therefore, it is an object of the present invention toprovide an optical direct amplifier device which can detect failure ofan excitation light source without using an electric circuit even when amonitoring system of the optical direct amplifier device based on a lineturning back is employed.

[0010] According to one aspect of the present invention, an opticaldirect amplifier device including first and second optical amplifiersrespectively amplifying signal lights of first and second transmissionpath, comprises:

[0011] a monitoring circuit branching only monitoring light wavelengthof different from a wavelength of the signal lights; and

[0012] extracting means for extracting a part of an excitation lightincided to the optical amplifiers.

[0013] Namely, the optical direct amplifier device of the presentinvention is characterized by a capability of performing failuredetection of the excitation light source of the high precision opticaldirect amplifier device in the optical direct amplifier system.

[0014] In the preferred construction, each of the first and secondoptical amplifiers which respectively amplifying signal lights of firstand second transmission path is formed with a series circuit of a rearearth added fiber, a wavelength division multiplexing coupler, anoptical isolator and an optical wavelength multiplexer/demultiplexer,and further includes a monitoring circuit for branching only monitoringlight wavelength different from the wavelength of the signal light andcombining it to the other transmission path, and the optical wavelengthmultiplexer/demultiplexer extracting a part of the excitation lightincided to the rear earth added fiber.

[0015] In the optical direct amplifier device according to the presentinvention, the rear earth added fiber amplifying the monitoring light isprovided in the monitoring circuit. The excitation light incided to therear earth added fiber is the excitation light extracted by the opticalwavelength multiplexer/demultiplexer. The optical direct amplifierdevice includes the wavelength division multiplexer for combining theexcitation light to the rear earth added fiber.

[0016] Here, when one of two laser diodes causes failure, an excitationlight power incided to the rear earth added fiber is reduced to make thepassing loss of the rear earth added fiber greater than the normal stateand thus to make lowering magnitude of the monitoring signal output ofthe optical direct amplifier device in the monitoring signal wavelengthgreater.

[0017] With the construction and operation set forth above, in theoptical direct amplifier system employing the monitoring system of theoptical direct amplifier device based on turning back of line, failureof the excitation light source of the optical direct amplifier devicecan be detected with high precision without using an electric circuit.

[0018] The monitoring circuit may include optical amplifying means fortaking a part of excitation light for the first and second opticalamplifiers extracted by the extracting means as an excitation lightsource, and amplifying the monitoring optical wavelength.

[0019] The optical direct amplifier device may further comprise meansfor coupling a part of the excitation light for the first and secondoptical amplifiers extracted by the extracting means, with the opticalamplifying means. The optical amplifying means may be set a gain innormal state of the excitation light source at less than or equal to 0dB.

[0020] The extracting means may use the excitation light leaking from anextra port of a coupler for multiplexing the excitation light of thefirst and second optical amplifiers.

[0021] The monitoring circuit may be constructed by connecting one ofports of a selective wavelength reflector selectively reflecting themonitoring light wavelength to the optical amplifying means andconnecting the other port of the selective wavelength reflector to theextracting means.

[0022] The optical direct amplifier device may further comprise meansprovided downstream side of the selective wavelength reflector, forconnecting the one of ports to the optical amplifier means andattenuating signal other than the monitoring light wavelength passingthrough the selective wavelength reflector. One light emitting elementof the excitation light for the first and second optical amplifiers maybe provided for each of the first and second transmission paths, and apart of the excitation light of the light emitting element is suppliedfor each of light amplifying means for the first and second transmissionpaths. In the alternative, a plurality of the light emitting elements ofthe excitation lights for the first and second optical amplifiers may beprovided for each of the first and second transmission paths, and partsof the excitation lights of the light emitting elements are supplied inplural for each of light amplifying means for the first and secondtransmission paths. In the further alternative, one light emittingelement of the excitation light for the first and second opticalamplifiers may be provided for each of the first and second transmissionpaths, and second wavelength division multiplexer is provided in each offirst and second transmission paths, a part of excitation light of thelight emitting element and a signal of the branched monitoring lightwavelength are combined by the second wavelength division multiplexer tosupply an output thereof to the optical amplifier means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be understood more fully from thedetailed description given hereinafter and from the accompanyingdrawings of the preferred embodiment of the present invention, which,however, should not be taken to be limitative to the invention, but arefor explanation and understanding only.

[0024] In the drawings:

[0025]FIG. 1 is a schematic block diagram of a basic circuit showing aconstruction of one embodiment of an optical direct amplifier deviceaccording to the present invention;

[0026]FIG. 2 is a schematic block diagram of a basic circuit showing aconstruction of another embodiment of an optical direct amplifier deviceaccording to the present invention;

[0027]FIG. 3 is a schematic block diagram of a basic circuit showing aconstruction of a further embodiment of an optical direct amplifierdevice according to the present invention;

[0028]FIG. 4 is a schematic block diagram of a basic circuit showing aconstruction of a still further embodiment of an optical directamplifier device according to the present invention; and

[0029]FIG. 5 is a schematic block diagram of a basic circuit showing theconstruction of the conventional optical direct amplifier device.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030] The present invention will be discussed hereinafter in detail interms of the preferred embodiment of an optical direct amplifier deviceaccording to the present invention with reference to the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be obvious, however, to those skilled in the art thatthe present invention may be practiced without these specific details.In other instance, well-known structure is not shown in detail in orderto avoid unnecessary obscurity of the present invention.

[0031]FIG. 1 is a schematic block diagram showing a basic circuitshowing a construction of one embodiment of an optical direct amplifierdevice according to the present invention. In FIG. 1, one embodiment ofthe optical direct amplifier device according to the invention includesrear earth added fibers 1 a, 1 b, 2 a and 2 b, WDM (Wavelength DivisionMultiplexing) couplers 3 a and 3 b, optical isolators 4 a and 4 b,optical wavelength multiplexers/demultiplexers 5 a, 5 b and 7, laserdiodes (LD) 6 a and 6 b, a control circuit 8, and monitoring signalreflectors 9 a and 9 b.

[0032] In one embodiment of the optical direct amplifier according tothe present invention, the rear earth added fiber 1 a, the WDN coupler 3a, the optical isolator 4 a, the optical wavelengthmultiplexer/demultiplexer 5 a are connected in sequential order from anup-link signal input end to reach an up-link signal output end.

[0033] Similarly, in one embodiment of the optical direct amplifieraccording to the present invention, the rear earth added fiber 1 b, theWDN coupler 3 b, the optical isolator 4 b, the optical wavelengthmultiplexer/demultiplexer 5 b are connected in sequential order from andown-link signal input end to reach an down-link signal output end.

[0034] Here, the WDM couplers 3 a and 3 b are couplers for inputting anexcitation light to the rear earth added fibers 1 a and 1 b. The laserdiodes 6 a and 6 b are connected to the control circuit 8 so that theoutputs thereof are controlled by the control circuit 8. The laserdiodes 6 a and 6 b are connected to two input ends of the opticalwavelength multiplexer/demultiplexer 7. Two output ends of the opticalwavelength multiplexer/demultiplexer 7 are connected to another inputend other than the foregoing input/output path.

[0035] On the other hand, in a monitoring circuit of the optical directamplifier device formed by connecting optical parts, such as the opticalwavelength multiplexers/demultiplexers 5 a and 5 b located downstreamside of the up-link optical isolator 4 a and the down-link opticalisolator 4 b, the rear earth added fibers 2 a and 2 b which are variableof gains depending upon excitation light power, and the monitoringsignal reflectors 9 a and 9 b formed by fiber grating or the like andlocated downstream side of the rear earth added fibers 2 a and 2 b, areconnected to branching ports of a signal light direction of the opticalwavelength multiplexers/demultiplexers 5 a and 5 b. On the other hand,by connecting the branching ports of the WDM couplers 3 a and 3 b inexciting direction to downstream side of respective monitoring signalreflectors 9 a and 9 b, the excitation light incides to the rear earthadded fibers 2 a and 2 b. Furthermore, the branching ports of theoptical wavelength multiplexers/demultiplexers 5 a and 5 b in ananti-signal light direction are connected with each other.

[0036] Operation of one embodiment of the optical direct amplifierdevice according to the present invention will be discussed withreference to FIG. 1. In one embodiment of the optical direct amplifierdevice according to the present invention, excitation light outputs fromthe laser diodes 6 a and 6 b are multiplexed and demultiplexed in theoptical wavelength multiplexer/demultiplexer 7 to incide to the up-linkrear earth added fiber 1 a and the down-link rear earth added fiber 1 bin an opposite direction to the signal. By this operation, the opticalsignal incident from the input end is amplified via the rear earth addedfibers 1 a and 1 b to be output from the output end. At this time,respective laser diodes 6 a and 6 b are controlled the outputs by thecontrol circuit 8.

[0037] The monitoring signal of the up-link system input from UP_IN sideis branched by the optical wavelength multiplexer/demultiplexer 5 a.After passing the rear earth added fiber 2 a, the branched monitoringsignal is reflected by the monitoring signal reflector 9 a and is outputfrom the DOWN_OUT via the optical wavelength multiplexer/demultiplexer 5b. The monitoring signal of the down-link system input from DOWN_IN sideis branched by the optical wavelength multiplexer/demultiplexer 5 b.After passing through the rear earth added fiber 2 b, the branchedmonitoring signal is reflected by the monitoring signal reflector 9 b tobe output from UP_OUT via the optical wavelengthmultiplexer/demultiplexer 5 a.

[0038] Among excitation light incided to the rear earth added fibers 1 aand 1 b, the excitation lights from the branching side from the WDMcouplers 3 a and 3 b respectively pass through the monitoring signalreflectors 9 a and 9 b to be incided to the ear earth added fibers 2 aand 2 b. By this operation, the rear earth added fibers 2 a and 2 bbecome optical amplifier. Gains of the branched excitation lights of therear earth added fibers 2 a and 2 b are preliminarily set certain value(e.g. 0 dB).

[0039] Here, consideration is given for the case where output of one oftwo laser diodes 6 a and 6 b becomes OFF due to failure. In this case,the excitation light output from the optical wavelengthmultiplexer/demultiplexer 7 is reduced. Accordingly, the excitationlights branched from the WDM couplers 3 a and 3 b are also reduced toreduce gains of the rear earth added fibers 2 a and 2 b (e.g. −3 dB).Thus, passing loss of the return path (for example, return path from theup-link line to the down-link line) in the monitoring circuit becomesgreater than that in the normal state.

[0040] Monitoring of the optical direct amplifier device in the opticaldirect amplifier system mounting the monitoring circuit based on turningback is performed employing a method, in which the monitoring signalfrom a not shown end station is transmitted to turn back a part of theoptical signal to the opposite side line in the optical direct amplifierdevice and to received the turned back monitoring signal at the endterminal. In general, the optical direct amplifier device causingabnormality in optical output is detected based on the loweringmagnitude of the monitoring signal output of the optical directamplifier device in the monitoring signal wavelength.

[0041] In one embodiment of the optical direct amplifier deviceaccording to the present invention, high precision failure detection ofthe optical direct amplifier device becomes possible for enlargedlowering magnitude of the monitoring signal output of the optical directamplifier device causing failure.

[0042] As set forth above, since the rear earth added fibers, means forbranching the excitation light and means for inciding the excitationlight are connected to the monitoring circuit of the optical directamplifier device, if failure is cased in one of a plurality of laserdiodes 6 a and 6 b, the excitation light power incided to the rear earthadded fibers in the monitoring circuit is reduced to make passing lossgreater than that in the normal state to enlarge or enhance the loweringmagnitude of the monitoring signal output of the optical directamplifier device by turning back. Therefore, high precision failuredetection of the excitation light source of the optical direct amplifierdevice in the optical direct amplifier system can be done by employingthe monitoring system of the optical direct amplifier device based online turning back.

[0043] On the other hand, comparing the conventional optical directamplifier device and one embodiment of the optical direct amplifierdevice of the present invention, number of increased parts numberbecomes two at the minimum (basically, the rear earth added fibers 2 aand 2 b) and addition of the electrical circuit becomes unnecessary,additional circuits required in connection with enhancing of precisionin failure detection of the optical direct amplifier device can bereduced.

[0044]FIG. 2 is a schematic block diagram of the basic circuit showingthe construction of another embodiment of the optical direct amplifierdevice according to the present invention. In FIG. 2, another embodimentof the optical direct amplifier device according to the presentinvention is similar in the construction as one embodiment of theoptical direct amplifier device according to the present inventionillustrated in FIG. 1 except that isolators 13 a and 13 b passing thesignal from the WDM couplers 3 a and 3 b only in the excitation incidentdirection is connected downstream of the monitoring signal reflectors 9a and 9 b. Like elements to those in the former embodiment will beidentified by like reference numerals and the construction and operationof such common elements are also similar to the former embodiment.Therefore, detailed description of such common elements will beeliminated for avoiding redundant disclosure and whereby to keep thedisclosure simple enough to facilitate clear understanding of thepresent invention.

[0045] In another embodiment of the optical direct amplifier deviceaccording to the present invention, the optical isolators 13 a and 13 bpassing the signals only in the incident direction of the excitationlight from the WDM couplers 3 a and 3 b are connected at positionsdownstream of the monitoring signal reflectors 9 a and 9 b.

[0046] By connecting these optical isolators 13 a and 13 b, the signalsother than the monitoring signal wavelength passing through themonitoring signal reflectors 9 a and 9 b can be attenuated. On the otherhand, the optical isolators 13 a and 13 b may be optical filters passingonly excitation lights. Another embodiment of the optical directamplifier device according to the present invention also achieves thesimilar effect as that achieved by the former embodiment of the presentinvention.

[0047]FIG. 3 is a schematic block diagram of the basic circuit showingthe construction of a further embodiment of the optical direct amplifierdevice according to the present invention. In FIG. 3, the furtherembodiment of the optical direct amplifier device according to thepresent invention multiplexes and demultiplexes the excitation lightoutputs from the laser diodes 6 a and 6 b by an optical wavelengthmultiplexer/demultiplexer 7 a. The multiplexed/demultiplexed output ofthe optical wavelength multiplexer/demultiplexer 7 a is incides to theup-link rear earth added fiber la and the down-link rear earth addedfiber 1 b from opposite direction as the signal by the WDM couplers 3 aand 3 b. On the other hand, the excitation light outputs from the laserdiodes 6 a and 6 b are multiplexed/demultiplexed by the opticalwavelength multiplexer/demultiplexer 7 b. The multiplexed/demultiplexedoutputs of the optical wavelength multiplexer/demultiplexers 7 a and 7 bare incided to the up-link rear earth added fiber 1 a and the down-linkrear earth added fiber 1 b from the same direction as the signal by theWDM couplers 11 a and 11 b. Except for those set forth above, the shownembodiment of the optical direct amplifier device is similar to oneembodiment of the present invention set forth above. Like elements tothose in the former embodiment will be identified by like referencenumerals and the construction and operation of such common elements arealso similar to the former embodiment. Therefore, detailed descriptionof such common elements will be eliminated for avoiding redundantdisclosure and whereby to keep the disclosure simple enough tofacilitate clear understanding of the present invention.

[0048] In the further embodiment of the optical direct amplifier deviceaccording to the present invention, the excitation light outputs fromthe laser diodes 6 a and 6 b are multiplexed and demultiplexed by theoptical wavelength multiplexer/demultiplexer 7 a. The multiplexed anddemultiplexed excitation light output from the optical wavelengthmultiplexer/demultiplexer 7 a is incided to the up-link rear earth addedfiber la and the down-link rear earth added fiber 1 b in oppositedirection to the signal in the WDM couplers 3 a and 3 b. On the otherhand, the excitation light output from the laser diodes 6 a and 6 b arealso multiplexed and demultiplexed by the optical wavelengthmultiplexer/demultiplexer 7 b and then incided to the up-link rear earthadded fiber 1 a and the down-link rear earth added fiber 1 b in the samedirection to the signal in the WDM couplers 11 a and 11 b.

[0049] With the operation set forth above, the optical signal incidedfrom the input end in the further embodiment of the optical directamplifier device according to the present invention, is amplifiedthrough the rear earth added fibers 1 a and 1 b and output from theoutput end. At this time, respective laser diodes 6 a and 6 b arecontrolled outputs by control circuits 8 a and 8 b.

[0050] The monitoring signal of the up-link system input from UP_IN sideis branched by the optical wavelength multiplexer/demultiplexer 5 a,passed through the rear earth added fiber 2 a, and then reflected by themonitoring signal reflector 9 a to be output from DOWN_OUT through theoptical wavelength multiplexer/demultiplexer 5 b. The monitoring signalof the down-link system input from DOWN_IN side is branched by theoptical wavelength multiplexer/demultiplexer 5 b, passed through therear earth added fiber 2 b, reflected by the monitoring signal reflector9 b and then output from UP_OUT through the optical wavelengthmultiplexer/demultiplexer 5 a.

[0051] On the other hand, among the excitation light incided to the rearearth added fibers 1 a and 1 b, the excitation lights from the branchedside of the WDM couplers 3 a, 11 a and the WDM couplers 3 b, 11 b aremultiplexed by respective optical wavelength multiplexers/demultiplexers12 a and 12 b and then incided to the rear earth added fibers 2 a and 2b through the monitoring signal reflectors 9 a and 9 b. By thisoperation, the rear earth added fibers 2 a and 2 b serve as an opticalamplifier. The gains of the rear earth added fibers 2 a and 2 b by thebranched excitation lights are preliminarily set to be a certain value(e.g. 0 dB).

[0052] Here, consideration is given for the case where failure is causedin one of four laser diodes 6 a to 6 d to turn the output thereof OFF.In this case, the excitation light outputs output from the opticalwavelength multiplexer/demultiplexer 7 a or the optical wavelengthmultiplexer/demultiplexer 7 b is lowered. Accordingly, the excitationlights branched from the WDM couplers 3 a, 3 b or the WDM couplers 11 a,11 b are also lowered to reduce the gains of the rear earth added fibers2 a and 2 b (e.g. −3 dB). As a result, the passing loss of the turningback path (for example, the turning back path from the up-link line tothe down-link line) in the monitoring circuit becomes greater than thatin the normal condition.

[0053] Similarly, when failure is caused in two of four laser diodes 6 ato 6 d to turn the outputs thereof OFF, the gains of the rear earthadded fibers 2 a and 2 b are reduced (e.g. −5 dB). On the other hand,when failure is caused in three of four laser diodes 6 a to 6 d to turnthe outputs thereof OFF, the gains of the rear earth added fibers 2 aand 2 b are further reduced (e.g. −6 dB) to make the passing loss of theturning back path in the monitoring circuit (for example, turning backfrom the up-link line to the down-link line) greater than normal state.

[0054] By constructing as set forth above, even when the excitationlight source becomes three or more, in the further embodiment of theoptical direct amplifier device, similar effect to the formerembodiments of the present invention can be achieved.

[0055]FIG. 4 is a block diagram of a basic circuit showing theconstruction of a still further embodiment of the optical directamplifier device according to the present invention. In FIG. 4, thestill further embodiment of the optical direct amplifier deviceaccording to the present invention branches the excitation light outputsfrom the laser diodes 6 a and 6 b by respective optical wavelengthmultiplexers/demultiplexers 15 a and 15 b to incide one of the branchedexcitation light output to the up-link rear earth added fiber la and thedown-link rear earth added fiber 1 b in the opposite direction to thesignal by respective WDM couplers 3 a and 3 b, and to incide the otherof the branched excitation light output to the rear earth added fibers 2a and 2 b of the monitoring circuit by respective WDM couplers 14 a and14 b. Other construction is the same as one embodiment of the presentinvention shown in FIG. 1. Like elements to those in the formerembodiment will be identified by like reference numerals and theconstruction and operation of such common elements are also similar tothe former embodiment. Therefore, detailed description of such commonelements will be eliminated for avoiding redundant disclosure andwhereby to keep the disclosure simple enough to facilitate clearunderstanding of the present invention.

[0056] In the still further aspect of the optical direct amplifierdevice according to the present invention, the excitation lightoutputted from the laser diodes 6 a and 6 b are demultiplexed byrespective optical wavelength multiplexers/demultiplexers 15 a and 15 bto incide one of the branched excitation light output to the up-linkrear earth added fiber la and the down-link rear earth added fiber 1 bin the opposite direction to the signal by respective WDM couplers 3 aand 3 b, and to incide the other of the branched excitation light outputto the rear earth added fibers 2 a and 2 b of the monitoring circuit byrespective WDM couplers 14 a and 14 b.

[0057] At this time, the outputs of respective laser diodes 6 a and 6 bare controlled by the control circuits 8 a and 8 b. The gain of theoptical amplifier in the monitoring circuit having the rear earth addedfibers 2 a and 2 b is preliminarily set at a certain value (e.g. 0 dB).

[0058] By constructing as set forth above, the still further embodimentof the optical direct amplifier device achieves similar effect to theformer embodiments of the present invention.

[0059] As set forth above, with the present invention, in the opticaldirect amplifier device including the first and second opticalamplifiers respectively amplifying the signal lights in the first andsecond transmission paths, failure detection of the excitation lightsource can be performed without using the electrical circuit even whenthe monitoring system of the optical direct amplifier device based online turning back, by providing the monitoring circuit branching onlythe monitoring light wavelength of the wavelength different from thesignal light and the extracting means for extracting a part of theexcitation light incided to the rear earth added fibers forming theoptical amplifier.

[0060] Although the present invention has been illustrated and describedwith respect to exemplary embodiment thereof, it should be understood bythose skilled in the art that the foregoing and various other changes,omission and additions may be made therein and thereto, withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention should not be understood as limited to thespecific embodiment set out above but to include all possibleembodiments which can be embodied within a scope encompassed andequivalent thereof with respect to the feature set out in the appendedclaims.

What is claimed is:
 1. An optical direct amplifier device includingfirst and second optical amplifiers respectively amplifying signallights of first and second transmission path, comprising: a monitoringcircuit branching only monitoring light wavelength of different from awavelength of said signal lights; and extracting means for extracting apart of an excitation light incided to said optical amplifiers.
 2. Anoptical direct amplifier device as set forth in claim 1, wherein each ofsaid first and second optical amplifiers is formed with a series circuitof a rear earth added fiber, a wavelength division multiplexing coupler,an optical isolator and an optical wavelength multiplexer/demultiplexer.3. An optical direct amplifier device as set forth in claim 1, whereinsaid monitoring circuit includes optical amplifying means for taking apart of excitation light for said first and second optical amplifiersextracted by said extracting means as an excitation light source, andamplifying said monitoring optical wavelength.
 4. An optical directamplifier device as set forth in claim 3, which further comprises meansfor coupling a part of the excitation light for said first and secondoptical amplifiers extracted by said extracting means, with said opticalamplifying means.
 5. An optical direct amplifier device as set forth inclaim 3, wherein said optical amplifying means is set a gain in normalstate of said excitation light source at less than or equal to 0 dB. 6.An optical direct amplifier device as set forth in claim 1, wherein saidextracting means uses the excitation light leaking from an extra port ofa coupler for multiplexing the excitation light of said first and secondoptical amplifiers.
 7. An optical direct amplifier device as set forthin claim 3, wherein said monitoring circuit is constructed by connectingone of ports of a selective wavelength reflector selectively reflectingsaid monitoring light wavelength to said optical amplifying means andconnecting the other port of said selective wavelength reflector to saidextracting means.
 8. An optical direct amplifier device as set forth inclaim 7, which further comprises means provided downstream side of saidselective wavelength reflector, for connecting said one of ports to saidoptical amplifier means and attenuating signal other than saidmonitoring light wavelength passing through said selective wavelengthreflector.
 9. An optical direct amplifier device as set forth in claim3, wherein one light emitting element of the excitation light for saidfirst and second optical amplifiers is provided for each of said firstand second transmission paths, and a part of said excitation light ofsaid light emitting element is supplied for each of light amplifyingmeans for said first and second transmission paths.
 10. An opticaldirect amplifier device as set forth in claim 3, wherein a plurality ofthe light emitting elements of the excitation lights for said first andsecond optical amplifiers are provided for each of said first and secondtransmission paths, and parts of said excitation lights of said lightemitting elements are supplied in plural for each of light amplifyingmeans for said first and second transmission paths.
 11. An opticaldirect amplifier device as set forth in claim 3, wherein one lightemitting element of the excitation light for said first and secondoptical amplifiers is provided for each of said first and secondtransmission paths, and second wavelength division multiplexer isprovided in each of first and second transmission paths, a part ofexcitation light of said light emitting element and a signal of saidbranched monitoring light wavelength are combined by said secondwavelength division multiplexer to supply an output thereof to saidoptical amplifier means.